1. Field of the Invention
The present invention generally relates to a band pass filter and a duplexer having the same. More particularly, the present invention relates to a band pass filter and a duplexer capable of improving filtering, and simultaneously achieving a high degree of integration and a reduction of manufacturing costs, by improving signal attenuation.
2. Description of the Related Art
Recently, as mobile communication devices such as mobile phones have become more widely used, there have been ongoing efforts to improve the performance of the mobile communication devices and to manufacture devices that are smaller and more lightweight. Accordingly, research into ways of achieving miniaturization and weight reduction is in progress through enhancement of the performance of components in the mobile communication device.
One of the essential components of a mobile communication device is a duplexer. A duplexer is a device that incorporates a filter. The duplexer permits efficient sharing of the same antenna by separating signals that are received and transmitted via one antenna in a communication system that is implemented using a frequency division duplex (FDD).
The basic structure of the duplexer includes a transmission filter and a reception filter, in addition to an antenna. The transmission filter is a band pass filter which passes a signal only within a frequency band to transmit the signal, and the reception filter is a band pass filter which passes a signal only within a frequency band to receive the signal. The duplexer permits transmission and reception via one antenna which adjusts the frequencies passed by the transmission filter and the reception filter.
The transmission filter and the reception filter, constructing the basic structure of the duplexer, can be implemented using a film bulk acoustic resonator (FBAR). The FBAR is small and lightweight and is known as a reliable means for configuring a filter for high power. An advantage of the FBAR is that it allows mass production with minimum cost, and can be compactly implemented.
In addition, the FBAR can realize a high quality factor (Q) value, which is an important characteristic of the filter, and can be used in a micro-frequency band. More advantageously, the FBAR can be implemented in personal communication system (PCS) and digital cordless system (DCS) bands.
The FBAR is manufactured so that a lower electrode, a piezoelectric layer, and an upper electrode are layered in order. When an external electrical field is applied, the FBAR generates resonance. In more detail, when electrical energy is applied to the upper and lower electrodes and the electrical field, which changes over time, is induced in the piezoelectric layer, resonance occurs, because the piezoelectric layer causes the piezoelectric effect which changes the electrical energy to mechanical energy of an acoustic wave form. In this case, because the FBAR passes only a signal within a specific band centering on the generated resonant frequency, it serves as the band pass filter.
FIG. 10 is a circuit diagram of a ladder type filter implemented by incorporating a plurality of FBARs in series and in parallel, as disclosed in U.S. Pat. No. 6,262,637.
Referring to FIG. 10, a conventional duplexer 10 comprises an antenna port 20, a transmission port 30, and a reception port 40. The duplexer 10 further comprises a transmission filter 50 between the antenna port 20 and the transmission port 30, a reception filter 60 between the antenna port 20 and the reception port 40, and a phase shifter 70 between the antenna port 20 and the reception filter 60.
The transmission filter 50 and the reception filter 60 comprise: a plurality of first resonators FBAR11 through FBAR16 that are connected between their coupled ports in series; a plurality of second resonators FBAR21 through FBAR26 that are formed on branching lines between the ports to which the transmission filter 50 and the reception filter 60 are connected, and connected to the first resonators FBAR11 though FBAR16 in parallel; and inductors IDT3 through IDT8, disposed between the second resonators FBAR21 through FBAR26 and a ground GND, and connected to the second resonators FBAR21 through FBAR 26 in series in order to regulate the frequency of the second resonators FBAR21 through FBAR26.
As described above, the plurality of first resonators FBAR11 through FBAR16 and the second resonators FBAR21 through FBAR26 are incorporated in series and in parallel to implement the ladder type filter. This functions as a band pass filter that filters signals in the specific frequency bands for transmission and reception.
Also, because the conventional duplexer 10 functions to properly separate the signals transmitted and received via one antenna, it is necessary to avoid interference between the transmitted signal and the received signal in order to improve the performance. To be more specific, because the frequency difference between the signals transmitted and received through the transmission filter and the reception filter is very small, the duplexer is sensitive to the signal interference. Therefore, it is necessary to prevent interference between the transmitted signal and the received signal.
In order to achieve this, the duplexer 10 generally requires an isolation part to prevent interference by isolating the transmission filter 50 and the reception filter 60. The performance of the duplexer 10 can be enhanced through the presence of the isolation part which prevents interference and noise insertion.
The isolation part typically implements the phase shifter 70 using a capacitor and an inductor to make the frequency phase difference of the transmitted and received signals 90 degrees. Thus, the isolation part can block the interference between the transmitted signal and the received signal.
As constructed above, the duplexer 10 needs to serially connect the inductors IDT3 through IDT8 to the second resonators FBAR21 through FBAR26 respectively to regulate the frequency of the transmission filter 50 and the reception filter 60, for example, to lower the resonant frequencies of the second resonators FBAR21 through FBAR26. In doing so, the inductors IDT3 through IDT8 employ high inductance of the 3˜4 nH frequency and high Q.
In conclusion, when the conventional duplexer is manufactured, the number of inductors used increases, which is disadvantageous to high integration and miniaturization. The use of the inductors with the high Q raises the manufacturing costs.